Spatial analysis and mapping in epidemiology have a long history but until recently, their use in public health has been limited. Maps were either created manually, or in research institutes using capital-intensive GIS hardware and software.
However, recent advances in geographical information and mapping technologies and increased awareness have created new opportunities for public health administrators to enhance their planning, analysis and monitoring capabilities. The late 1990s have seen a significant expansion in information and mapping technology, including the development of desktop mapping software, new programming tools for customization of mapping products and increasing connectivity to information highways such as the World Wide Web.
There are many definitions of GIS. GIS are often described as an organized collection of computer hardware, software, geographical data and personnel designed to efficiently capture, store, update, manipulate, analyse and display all forms of geographically referenced information. While accurate, comprehensive and quite widely accepted, this definition may not help the public health newcomer to GIS. They are first and foremost an information system with a geographical variable which enable users to easily process, visualize and analyse their data or information spatially. Each piece of information is related in the system through specific geographical coordinates (e.g. latitude and longitude) to a geographical context. This can be a health facility, a laboratory, a village, a district, a region, a country or a group of countries. The information can be displayed in the form of graphs, charts and maps, although GIS are mainly used to display results in the form of maps.
Use of GIS in epidemiological surveillance
Geographical information systems and maps are valuable in strengthening the whole process of epidemiological surveillance information management and analyses.
In data collection
• A GIS provides an excellent means of collecting, updating and managing epidemiological surveillance and related information. A GIS can store, handle and geographically integrate large amounts of information from different sources, programmes and sectors.
In data management
• A GIS serves as a common platform for convergence of multidisease surveillance activities. Standardized georeferencing of epidemiological data facilitates standardized approaches to data management. As such, a GIS can serve as an entry point for integrating disease surveillance activities where appropriate.
• A GIS facilitates the convergence of multisectoral data, including epidemiological surveillance information, population information, environmental information and health and other resources into a common platform for analyses (Figure 1).
Figure 1: This figure illustrates how Guinea worm eradication programmes use GIS for standardized data entry and updating of epidemiological and programmatic data. National Coordinators can track all villages under surveillance, quickly identify newly infected and reinfected villages and monitor progress of eradication of the disease. The system contains all historical information to support the required procedures for eventual certification of eradication of the disease.
In data analysis
• A GIS provides an excellent means of visualizing and analysing epidemiological data, thus revealing trends, dependencies and interrelationships that would be more difficult to discover in other formats (Map1).
Questions a GIS can answer
There are basically 5 types:
Condition: What is...?
Location: Where is it?
Trends: What has changed since...?
Patterns: What spatial patterns exist?
Modelling: What if...?
Functions of a GIS
A GIS can help answer specific questions and perform the following functions:
• generate "thematic" maps (ranged colour maps or proportional symbol maps to denote the intensity of a mapped variable);
• allow for overlaying of different pieces of information;
• create buffer areas around selected features (for example a radius of 10 km around a health centre to denote a catchment area or 1 km around a water point or school);
• carry out specific calculations (the proportion of the population falling within a certain radius of a health facility, school, dam etc.);
• calculate distances (e.g. the distance of a community to a health facility) (Map 2);
• permit a dynamic link between databases and maps so that data updates are automatically reflected on the maps;
• permit interactive queries of information contained within the map, table or graph;
• process images such as aerial or satellite images to allow information such as temperature, rainfall, soil types and land use to be easily integrated, and spatial correlations between potential risk factors and the occurrence of diseases to be determined;
• provide a range of extrapolation techniques (for example, extrapolating sentinel site surveillance to unsampled areas).
Map 1: A GIS can easily combine information from different countries to support cross-border monitoring of transmission of diseases. Here, some foci for African trypanosomiasis are clearly identified.
Map 2: As this example from Mali shows, GIS and mapping can be used to assess the adequacy of health services to respond to disease notification.
Use of GIS in public health
GIS and mapping technologies are being used by a wide variety of public health administrators, including policy makers, national programme managers, statisticians, epidemiologists, regional and district medical officers.
Sample GIS applications in public health
• Determining geographical distribution and variation of diseases (prevalence, incidence)
• Analysing spatial and longitudinal trends
• Mapping populations at risk
• Stratifying risk factors
• Assessing resource allocation (health services, schools, water points)
• Planning and targeting interventions
• Forecasting epidemics
• Monitoring diseases and interventions over time
First steps in using GIS
In order to establish an operational GIS for epidemiological surveillance, the following steps should be followed.
Determine the objectives of the GIS. Why do you want to use a GIS? What is the problem to be solved? What kinds of analysis are to be carried out? What are the final products expected of the GIS? Who is to access the GIS?
Access digitized basemaps, e.g. maps of administrative boundaries, rivers, roads, etc. that contain xy coordinates and are available as computerized files.
Georeference epidemiological surveillance datasets. Assigning a unique and standardized code or nomenclature to the geographical area in which you want to work (e.g. region, district, village, health centre). The georeference of a district must correspond to the digitized base map. The georeference of a village or health facility must be the exact geographical coordinates (latitude and longitude). When these do not already exist in the country, global positioning systems (GPS) can be used. A GPS can be used to obtain the geographical coordinates of a point on a map, such as a village, health facility or dam. A GPS is a hand-held receiver used to find a location in the field through radio transmission to satellites.
Choose the appropriate GIS software packages. There are many different commercial and non-commercial GIS software packages. The choice of software should be guided by specific GIS requirements, as each software offers different functions.
Acquire suitable computer hardware. The minimum requirement to run any GIS software is a suitable environment to operate a microprocessor with 32 MB Ram minimum, 30 MB of available space on hard disk, Windows 95/NT or 98.
The WHO/UNICEF Joint Programme on Health Mapping (Health Map) has developed a database management and mapping system called the HealthMapper that has been customized for public health applications at country, regional and global levels. The system contains a standardized georeferenced database of country, regional, district and subdistrict boundary maps, rivers, roads, villages, and health and social infrastructures. The system also comprises a user-friendly mapping interface and a database management interface. It is currently being used in West Africa and will be extended for use in all of Africa, South-East Asia and the Eastern Mediterranean regions of WHO.
For more information on how to get started using GIS for epidemiological surveillance and for accessing digitized basemaps, standardized geocoding methods, and the HealthMapper, please contact:
WHO/UNICEF Joint Programme on Data Management and Mapping - HealthMap
Communicable Disease Surveillance and Response (CSR)
World Health Organization, 20 Avenue Appia, 1211 Geneva 27, Switzerland
E-mail: email@example.com / Surveillancekit@who.ch
Tel: (41 22) 791 3881/3836
Fax: (41 22) 791 4198